This thread on the Rogers Community Forums explores the performance problems experienced by World of Warcraft players who use this Canadian telecom company as their ISP (thanks Blizzplanet via Nate). A post from Rogers confirms suspicions that they are throttling WoW connections, saying this is inadvertent, calling it "a problem with our traffic management equipment." They say a software update has helped a bit, and they are expecting to clear this up with another software update in June. In the meantime, they offer this workaround:

We believe the problem occurs when P2P is running while simultaneously playing the game. If you are experiencing problems we suggest you turn off the peer to peer setting within the WoW game and ensure no other P2P file sharing applications are running while playing WoW. WoW does use P2P for software updates, but with this setting changed you should continue to automatically receive software updates through other methods.

This is only a temporary solution. We continue to work closely with the game manufacturer and our equipment supplier to help resolve this issue as soon as possible.